1. Field of the Invention
The present invention relates to a stand-alone measuring apparatus capable of measuring stray currents from subway rails and power lines and the pipe-to-soil voltage of a buried metallic structure, and more particularly, but not by way of limitation, to a measuring apparatus with storage that allows simultaneous analysis of the relation between stray current from subway rails and the resulting pipe-to-soil voltage of a buried metallic structure and the relation between stray current from power lines and the resulting pipe-to-soil voltage of a buried metallic structure.
2. Description of the Related Art
Corrosion is the chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the metal and its properties. Corrosion of a metal is often called electrochemical corrosion because it is caused by an electrochemical reaction involving a flow of electrons between cathodic and anodic areas.
The electrochemical corrosion arises from naturally occurring processes at specific locations on a metallic structure involving electrical current flow into the ambient soil electrolyte from anodes where oxidation reactions occur. The current flows to sites on the structure acting as cathodes where reduction reactions occur. Four components of the electrochemical corrosion are an anode, a cathode, an electrical path or metallic path, and an ionic path or electrolyte.
Methods for detecting corrosion of a metal include the sonic reflection method, the ultrasonic test method, the LPR (linear polarization resistance)method, the ER (electrical resistance) method, and the electrochemical potential measurement method. The sonic reflection method locates cracks or corrosion of a metal by collecting a sonic beam reflected by the metal using an array sensor and analyzing the collected data. The ultrasonic test method inspects corrosion of a metal by detecting changes in the thickness of the metal reduced by the corrosion. The LPR method detects the instantaneous corrosion rate of a metal by inserting a test probe into a conductive fluid and measuring the polarization resistance. The ER method measures the corrosion rate of a metal by inspecting changes in the electrical resistance of the metal caused by long-term electrochemical corrosion. The electrochemical potential measurement method detects the corrosion of a metal by measuring the electrochemical potential of the metal. The electrochemical potential measurement method is the most widely used method.
The electrochemical potential measurement method involves the measurement of the electrochemical potential difference between a buried metallic structure, which is a working electrode, and a reference electrode (Cu/CuSO4, CSE) in an electrical contact with ground above the structure, which is an electrolyte. The potential is measured with the positive and negative terminals of a measuring instrument connected to the reference electrode and the metallic structure respectively and then the sign of the measured value is reversed. The potential value is used to determine the level of the electrochemical corrosion. For example, a potential (e.g., −1000 mV) less than the cathodic protection criteria (−850 [mV/CSE], 250 [mV/Zn] in case of a zinc reference electrode) indicates that the metallic structure is protected from the corrosion. A potential greater than the cathodic protection criteria indicates that the metallic structure is corroding away.
Conventionally, to check the status of buried or submerged structures such as gas pipelines, tanks, and water pipelines, corrosion inspection of such structures has been conducted on a regular/irregular basis.
The corrosion inspection is conducted intermittently by a human inspector using a portable tester or a portable strip chart recorder. A shortcoming of such a corrosion inspection is that it takes lots of time because the human inspector need to connect test leads to a test box, the location of which is normally on the street, for the corrosion potential measurement (−terminal to the pipe and +terminal to the reference electrode) and to move to a position which allows easy and safe measurement.
For the reason, a corrosion inspection instrument that can be inserted into the test box for the corrosion potential measurement, which was developed by the inventor of the present invention, has been used recently.
Corrosion protection generally means the elimination of one or more sources that cause the corrosion. Because it is practically impossible to eliminate all the sources of the corrosion completely, methods that can mitigate the electrochemical corrosion reaction using inhibitors, insulators, etc are commonly employed. Cathodic protection is one of the most popular corrosion protection methods.
Cathodic protection provides corrosion protection to any bare metal areas exposed to soil by causing direct current (DC) to flow from the soil into the metallic structure, thereby polarizing the metallic structure as a cathode.
It is possible to cathodically protect a metallic structure by an external power supply such as a rectifier that generates an impressed current through the soil from an anode to the protected object. The rectifier provides direct current (DC) to the protected structure in order to maintain the electrochemical potential of the protected structure below the cathodic protection criteria (e.g., −850 mV/CSE). In other words, if the potential difference between a buried metallic structure and the reference electrode measured by a portable tester or a portable strip chart recorder is greater than the cathodic protection criteria, the rectifier provides direct current (DC) to the protected structure through an insoluble anode (high silicon cast iron (HSCI)) and the soil, so that the potential difference may remain below the cathodic protection criteria to protect the metallic structure from the electrochemical corrosion.
If interference by stray currents from subway rails or power lines occurs, the pipe-to-soil voltage will fluctuate as shown in FIG. 6 or the third harmonic will be found as shown in FIG. 7, respectively. The subway power system is designed such that the current is provided to subway trains from a subway power substation through power feeder cables and the current returns to the subway power substation through rails after driving the subway trains.
In the subway power system, a portion of the current deviates from the designed path and leaks into the soil because of the longitudinal resistance of the rails and incomplete insulation between the rails and the soil. The current flowing into the soil is called stray current.
The stray current flows along the length of a metallic structure buried in the soil (e.g., gas pipeline, oil pipeline, water pipeline) acting as a conductor and leaks into the soil at some specific locations which are near the cathode or have low soil resistivity. The leakage current returns to the negative feeder of the subway power supplying system. Corrosion occurs intensively around such locations.
The corrosion caused by stray currents from the subway is called stray current corrosion or electrolytic corrosion. With a view to preventing the stray current corrosion, the magnitude of the interference and the positions in which the interference occurs are analyzed by measuring only the pipe-to-soil voltage of a buried metallic structure.
Stray currents from power lines are affective by various types of induction such as capacitive induction, inductive induction, and resistive induction. AC induction voltage caused by resistive induction is dominant at places where the aforementioned apparatuses for measuring the pipe-to-soil voltage are employed.
There is a resistive coupling effect between a grounding structures (rods, wires, mesh) and a buried metallic structure sharing the same electrolyte (soil), whereby energy can be transmitted between the two structures in the form of AC voltage or current. In an electrical power system having a grounded neutral wire, unbalanced current due to power system unbalanced or third harmonic currents may flow along the neutral wire. In this case, the energy is transmitted from the ground point to the buried metallic structure through the soil. If the buried metallic structure is coated with an insulator, a resistive induction voltage is generated between the metallic structure and the ground point.
Suppose that a grounding conductor of the power system is a hemisphere of radius r as shown in FIG. 9. A voltage inducted at a distance of x from O (the center of the hemisphere) when current I flows into the soil having a constant resistivity can be computed as follows.
Ohm's law states that the induced voltage v is expressed byv=IR  (1)
and since R=∫dR=∫ρ(dr/2π r2), the induced voltage v is equivalently given byv=ρI/2πx  (2)
The induced voltage v expressed by the equation (2) is induced at a pipe located at a distance of x from the grounding conductor providing current I to the soil.
As the equation (2) indicates, the magnitude of the induced voltage depends on the resistivity of the soil and the distance between the grounding conductor and the pipe if the current flowing into the soil through the ground remains constant.
In Korean power distribution systems, in which the primary voltage is 22,900 V, a 3-phase 4-wire system with multiple grounding of the neural wire is employed as shown in FIG. 10. Therefore, the problematic AC stray current is caused by the multiple grounding wires at a neutral point.
In the 3-phase 4-wire system, the fourth wire, i.e., the neutral wire is repeatedly grounded at regular intervals and is designed such that 60 Hz current does not flow through the neutral wire. However, in the case where a third harmonic component is generated due to an unbalanced load condition, some of the third harmonic current flows through the neutral wire and some of the harmonic current flows through the ground.
Because a buried pipeline is a good grounding conductor, the third harmonic current tends to flow into the pipe by the resistive coupling effect. The current inflow depolarizes the polarization on the surface of the pipe and increases the consumption rate of Mg anodes. Moreover, it may give an electric shock to pipeline workers and cause an explosion accident by an electric arc in case of an accidental arc.
For these reasons, the pipe-to-soil voltage of a buried metallic structure is measured on a regular basis using a portable measuring instrument or a corrosion inspection system developed by the inventor of the present invention to inspect whether the metallic structure is protected from corrosion.
Unfortunately, however, the apparatuses used for corrosion inspection were not capable of directly measuring the stray current and thus it was impossible to analyze the stray current.
It is possible to measure the stray current using an oscilloscope. To use an oscilloscope for the stray current measurement, however, the oscilloscope needs to be installed at a subway station while the subway is out of service and the measurement data needs to be collected after the measurement is completed.
However, oscilloscopes are expensive and not easy to install around subway rails because oscilloscopes are relatively big in size. To measure the stray current, it is required to install a plurality of oscilloscopes in multiple measurement points, which is almost impossible for practical reasons.
In conventional corrosion inspection methods which simply measure the pipe-to-soil voltage of a buried metallic structure, the positions where current leakage occurs and the magnitude of the stray current cannot be precisely detected.
Consequently, a small-sized and low-cost stand-alone measurement apparatus that can easily measure the stray currents from subway rails at a rail impedance bond and analyze the stray current is quite in need.